This invention relates to a variable pitch propeller control system, and more particularly, to a variable pitch propeller and control system including a main pitch control for normally operating the pitch change mechanism of a variable pitch propeller along with a secondary control for operating the pitch change mechanism as a back up to the main control and which does not rely upon flyweight mechanisms or the like as part of the second control.
Conventional variable pitch control systems for propellers employed in aircraft typically include a main control having two channels for redundancy which normally operate a pitch control mechanism in the propeller and a flyweight mechanism that acts as a back up or second control for the main control system. In systems of this sort, the flyweight mechanism is rotated with the propeller and as propeller angular velocity changes, the flyweights change their position in proportion to the angular velocity of the propeller. In the usual case, a spring or the like biases the flyweights toward one position and as the propeller angular velocity increases, the flyweights move against the bias of the spring. This movement is conventionally conveyed to a hydraulic valve which controls the flow of hydraulic fluid to a pitch change mechanism incorporated in the propeller itself. Should an undesirable propeller condition occur, such as an overspeed condition or movement of the propeller blades to a finer pitch than a so-called xe2x80x9cflight idlexe2x80x9d position, whereat the pitch of the blades is finer than a coarse position necessary to sustain flight, the valve controlled by the flyweight mechanism will direct hydraulic fluid at a greater pressure to the pitch change mechanism to move the blades toward a more coarse position, i.e., move the blades toward a feathered position, to alleviate the undesirable condition.
Such systems, like any other, require periodic maintenance to assure that they are operating properly, which is to say, to assure that they become operational at the proper threshold of propeller speed and/or flight idle pitch, amongst other things. At the same time, they must be adjusted so they do not interfere with the operational characteristics dictated by operation of the main control.
Experience with such systems in the field has tended to show that maintenance personnel working on such systems were spending more time adjusting and xe2x80x9ctweakingxe2x80x9d the mechanical back-up system, that is, the flyweight control, than on maintaining the main controls. This is not to say that maintenance of the main control is neglected, but rather, only that the costs of maintenance are undesirably large as a result of the effort to continually and properly adjust the secondary control system including a flyweight mechanism.
The present invention is directed to overcoming one or more of the above problems.
It is the principal object of the invention to provide a new and improved variable pitch propeller control system. More specifically, it is an object of the invention to provide a variable pitch propeller control system with a secondary control that eliminates mechanical components, such as a flyweight mechanism, that require an inordinate amount of time being maintained or adjusted to achieve proper balance in the system and compatibility with the main control.
An exemplary embodiment of the invention achieves the foregoing objects in a construction that includes a propeller having a hub rotatable about a rotational axis, and at least two propeller blades journal ed in the hub for rotation about axes crossing the rotational axis for blade pitch control as well as rotation about the rotational axis with the hub. The system includes a pitch change mechanism including a motor for rotating the blades in unison about the crossing axes between at least a coarse pitch position and a fine pitch position. A main pitch control is provided for normally operating the pitch change mechanism. There is further included a secondary control for operating the pitch change mechanism in response to the existence of at least one undesirable propeller condition and includes a target carried by the propeller along with a target pickup for sensing the target and generating a target signal. A function generator receives the target signal and generates at least one propeller condition signal indicating when at least one undesirable propeller condition occurs. A logic device is responsive to the propeller condition signal and is connected to the pitch change If mechanism and causes the pitch change mechanism to change the pitch of the blades to a pitch whereat the undesirable propeller condition no longer exists.
In a preferred embodiment, the main pitch control includes two control channels and the logic device causes the pitch change mechanism to change the pitch of the blades to a more coarse position.
In one embodiment of the invention, the undesirable propeller condition is a propeller overspeed condition and the function generator generates the propeller condition signal when the angular velocity of the propeller about the rotational axis exceeds a predetermined value.
According to one embodiment of the invention, the undesirable propeller condition is a propeller blade pitch less than a flight idle pitch and the function generator generates the propeller condition signal when the pitch of the blades about the crossing axes is a fine pitch finer than a predetermined flight idle pitch.
In a highly preferred embodiment, there are two undesirable propeller conditions including a propeller overspeed condition and a propeller blade pitch less than a flight idle pitch and the function generator generates the propeller condition signal when either or both 1) the angular velocity of the propeller about the rotational axis exceeds a predetermined value and/or 2) the pitch of the blades about the crossing axes is a fine pitch finer than a predetermined fine idle pitch.
In a preferred embodiment, the logic device includes a series of logic gates.
In a preferred embodiment, there is further included a device for selectively disabling the secondary control as, for example, during ground operations.
In a preferred embodiment, the secondary control includes two separate, generally identical control channels, each including one of the function generators and one of the logic devices.
Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings.